The present invention relates to a heat-sensitive recording material, more particularly, to one that has a high degree of whiteness, that is improved in recording characteristics such as color forming ability and that exhibit good keeping quality.
Heat-sensitive recording materials that rely upon the thermal color forming reaction between at least one colorless or pale-colored basic dye and a developer such as a phenol or an organic acid have been reported in many prior patents including Japanese Patent Publication No. 14039/1970 and some of these materials are extensively used in practical applications such as measuring recorders, computers, terminals of information communications equipment, facsimiles and automatic ticket vendors. The apparatus with which heat-sensitive recording materials can be used are equipped with heat-generating elements such as a thermal head and a thermal pen, which are heated to produce a recording color density. With the recent diversification of uses and sophistication of apparatus, it has become necessary that heat-sensitive recording materials have even higher performance. For instance, at least several minutes were previously required to perform recording on sheets of a size A4 which facsimiles but it is now possible to reduce this period to less than one minute. Under these circumstances, the use of a sensitizer has been proposed as a method of enhancing the sensitivity of heat-sensitive recording materials but in fact, no sensitizer has yet been discovered that attains balance between the various characteristics required of heat-sensitive recording materials such as sensitivity for color formation, image stability and whiteness.
Heat-sensitive recording materials contain the fine particles of a color former and a phenolic compound dispersed in a heat-sensitive recording layer, and either one of the color former and the phenolic compound or both are melted by heating so that they are brought into close contact with each other to form a color. While several phenolic compounds are suitable for this purpose, 2,2-bis(4'-hydroxyphenyl)propane (bisphenol A with m.p. of 156.degree. C.) is extensively used on account of its low price and stable quality (see, for example, U.S. Pat. No. 3,539,375). However, bisphenol A is not quick in thermal response and has not always been capable of meeting the market need for increasing the sensitivity of heat-sensitive recording materials in response to the faster operation or size reduction of thermal recording equipment. In order to compensate for this shortcoming, various sensitizers have been proposed such as waxes (Japanese Patent Public Disclosure No. 19231/1973), phthalate esters (Japanese Patent Public Disclosure Nos. 116690/1982 and 98285/1973), benzyl phenyl ether derivatives (Japanese Patent Public Disclosure No. 155096/1984), benzyl biphenyl (Japanese Patent Public Disclosure No. 82382/1985), naphtholic derivatives (Japanese Patent Public Disclosure No. 87094/1983) and carbonic acid diesters (Japanese Patent Public Disclosure No. 136489/1983). However, the combinations of these sensitizers with bisphenol A have not yet succeeded in providing satisfactory quality in such aspects as whiteness, sensitivity for color formation and fogging. Japanese Patent Public Disclosure No. 144193/1981 proposed the use of benzyl p-hydroxybenzoate or methylbenzyl p-hydroxybenzoate as a developer intended to enhance the sensitivity of heat-sensitive recording sheets. However, this idea has turned out to be unsatisfactory for the purpose of insuring good keeping quality since white spots appeared in the area of color formation during the storage that followed the recording operation.
Thus, none of the heat-sensitive recording materials proposed so far have yet fully satisfied the rigorous requirements of the market for higher quality including good characteristics of color formation and long keeping quality.
The present inventors previously filed Japanese Patent Application No. 4498/1988 on the basis of the finding that a heat-sensitive recording material having high sensitivity and whiteness could be provided by using a dihydroxydiphenyl ether of the formula shown below as a developer: ##STR3## However, this compound proved to be unsatisfactory in terms of keeping quality. For instance, when the heat-sensitive recording material using this compound was left to stand at 60.degree. C. and 20% R.H. or at 40.degree. C. and 90% R.H. for 24 hours after recording, the image density decreased, sometimes to a level less than 80% of the initial density.
Hence, the present inventors continued their efforts to produce a heat-sensitive recording material that had both high sensitivity and good keeping quality. As a result, they found that not only high sensitivity and whiteness but also good keeping quality could be achieved by using the developer dihydroxydiphenyl ether of the formula (I) in combination with at least one compound selected from the group consisting of 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanuric acid and 1,1-bis(2-methyl-4-hydroxy-5-t-butylphenyl)butane. Based on this finding, the present inventors filed Japanese Patent Application No. 187338/1988.
In order to achieve further improvements in sensitivity and keeping quality, the present inventors continued their research work and found that a heat-sensitive recording material that had good keeping quality and that was more sensitive than the recording material taught in Japanese Patent Application No. 187338/1988 could be obtained by using the developer dihydroxydiphenyl ether (I) in combination with at least one compound selected from the group consisting of 1,1-bis(4-hydroxyphenyl)cyclohexane and a compound represented by the general formula: ##STR4## Based on this finding, the present inventors filed Japanese Patent Application No. 20477/1989.